1. Field of the Invention
This invention relates to elongate power transmission belts having at least one rib extending lengthwise of the belt.
2. Background Art
The ever increasing demand for energy savings has caused the proliferation of compact automobiles. Compacting of automobiles has resulted in the engines thereon being operated in relatively small compartments. As a result, the power transmission belts used in this environment are commonly subjected to high operating temperatures.
It is known to make power transmission belts using natural rubber, styrenebutadiene rubber and chloroprene rubber. In the high temperature environment of the engine compartment, the rubber in the compression section of the belts is prone to cracking at an early stage in the anticipated belt life.
To address this problem, improvements in chloroprene rubber have been investigated, with some improvements having already been made. However, so long as chloroprene rubber is used, there is a limit to the amount of improvement that can be made in this regard. There exists a need for further improvement.
As an alternative, the use of a rubber wherein the main chain is highly or completely saturated, such as chlorosulfonated polyethylene rubber, hydrogenated nitrile rubber, fluorine rubber, etc. has been investigated. These rubbers have excellent heat resistance. Of these rubbers, chlorosulfonated polyethylene rubber is generally the same as chloroprene rubber in terms of its dynamic fatigue, abrasion resistance, and oil resistance. Vulcanization, and in particular, the use of an acid acceptor improves water resistance. Typically, as an acid acceptor for chlorosulfonated ethylene rubber, an oxide such as MgO, PbO, or the like, is used.
However, while an acid acceptor made up of a lead compound such as PbO, Pb.sub.3 O.sub.4, etc. enhances water resistance, the use of a lead compound is undesirable because of environmental concerns and hygiene. If MgO is used as the acid acceptor, water resistance is greatly deteriorated with MgCl.sub.2 formed during the cross-linking reaction. Thus, the application of MgO to a belt is not practical. On the other hand, when an epoxy-based acid acceptor is used as an acid acceptor instead of metal oxides, a composition having good water resistance can be obtained. However, the product produces an unpleasant smell, making it unpleasant for those exposed to the product.
Power transmission belts using a chlorosulfonated polyethylene rubber have a longer running life in high temperature environments and better heat resistance compared to belts using chloroprene rubber. However, the running life of the power transmission belt using a chlorosulfonated polyethylene rubber deteriorates greatly in a low temperature environment i.e. on the order of -30.degree. C. or lower. It is presumed that this shortcoming is attributable to the fact that because conventional chlorosulfonated rubber is obtained by chlorosulfonating polyethylene and contains chlorine, the aggregation energy of chlorine becomes high at a low temperature. Curing of the rubber occurs at a low temperature to reduce the rubber elasticity, with the rubber being prone to becoming cracked.
On the other hand, an ethylene-.alpha.-olefin elastomer such as an ethylene-propylene-based rubber (EPR), an ethylene-propylene-diene-based rubber (EPDM), etc. has excellent heat resistance and cold resistance and is a relatively inexpensive polymer. However, because the elastomers do not have good oil resistance, these rubbers have not been used in environments in which they will be exposed to oil. For example, in dry frictional transmission such as with a V-ribbed belt, when a large amount of oil is applied, the belt tends to slip, making it impractical for this use. Use of such a belt has been investigated and is disclosed in, for example, JP-A-6-345948.
The ethylene-propylene-based rubber generally has a low tearing strength. When a peroxide-based cross-linking system is used, the tearing strength lowers even further, as a result of which there may be a problem with load carrying cords popping out during running.
On the other hand, with the ethylene-propylene-based rubber using a sulfur-base cross-linking system, because it is difficult to sufficiently increase the degree of vulcanization, belt abrasion during running increases. In the case of V-ribbed belts, abraded powders may accumulate at the root between adjacent ribs. Sticking abrasion may occur, which may generate unwanted noise.
Also, with EPDM having a very large number of double bonds in the molecules is used to increase the degree of vulcanization, the problem of the sticking abrasion can be alleviated to a certain extent, however, this has resulted in a lowering of the heat resistance.